4o6 



Journal of Agricultural Research 



Vol. Ul, No. 5 



in 24 hours the same as the control tubes. In Table III we have recorded 

 the percentage of cultures which gave a positive reaction in both of the 

 litmus-milk tubes and in only one milk tube after different periods of 

 incubation and when heated at different temperatures. 



Table III. — Effect of heat in relation to the time required for cultures to show grouth 



Hours of incubation. 



34- 



48. 



72. 



96. 



Total. 



Tube reaction. 



[2 tubes-)- . 



1 1 tube-f . . 

 [i tube—. . 



[2 tubes-|- . 



H tube-|-. . 

 [i tube — . . 



[2 tubes-|- . 



[ I tube-(- . . 

 [i tube— . . 



(2 tubes-l- . 



1 1 tube-f. . 

 1 1 tube— . . 



S4.S°C. 



Per cent. 



92-75 



5.80 

 1-45 



.«.2°C. 



(135° F.)- 



Per cent. 



28.34 

 14. 17 

 24.41 



12-59 

 5-52 



11.03 

 o 

 3-94 



60° c. 

 (.40° F.). 



Per cent. 

 II. II 



26.98 



17. 46 



20.63 



3- 17 

 19. 06 

 o 

 1-59 



6;.8° C. 

 (-45° F.). 



Per cent. 

 12.50 



62. 50 



25. 00 



From the table it will be seen that 92.75 per cent of the cultures 

 showed a positive reaction after 24 hours' incubation in both of the 

 duplicate tubes when heated at 54.5° C. (130° F.). After 48 hours' 

 incubation 5.8 per cent more of the cultures showed a positive reac- 

 tion in both tubes, while 1.45 per cent showed a positive reaction in 

 only one of the two tubes. At 62.8° C. (145° F.), however, only a small 

 percentage of the cultures were positive after 24 hours. The majority 

 required from 48 to 72 hours' incubation to show growth. This shows 

 that a large proportion of the cells were destroyed so that a longer incu- 

 bation was necessary to allow bacterial increases sufficient to cause a 

 positive reaction. These facts are further supported by the differences 

 in the number of cultures in which both duplicate tubes showed a positive 

 reaction. At 62.8° C. (145° F.) only a small percentage of the cultures 

 showed a positive reaction in both tubes, showing that in many cases 

 all the bacteria in one tube were destroyed, while in the duplicate tube 

 a few cells only survived. 



It is of interest to note that the colon bacilli are less heat-resistant 

 than the streptococci, as is shown in a previous paper (2). 



